Many electronic devices draw high instantaneous input currents when first switched on. Those currents are referred to as inrush currents and they not only complicate the design of, for example, overcurrent protection devices within the electronic device, but they may also cause actual damage to the electronic device as those inrush currents may exceed the normal steady state operating current of the electronic device by orders of magnitude.
One of the fields of electronics where high inrush currents may have to be dealt with are airbag systems. Airbag systems used nowadays mostly include an energy reserve capacitor which may be used to store charges for the case when for some reason, for example due to a crash, the electrical connection to the vehicle battery is lost. Therefore, the capacitance of the reserve capacitor is usually quite large, for example it may be a few tens of millifarad, for example 10 mF, in the exemplary case of a charge amounting to 330 millicoulomb which is stored at an operating voltage of 33V. With a reserve capacitor dimensioned properly, the airbag module may thus be provided with energy therefrom and be able to continue its proper operation. Thus the airbag may be deployed even when the connection to the battery is lost in case of an accident. However, the presence of the reserve capacitor with its big capacitance may be problematic with respect to the inrush current which may occur when the ignition of the car is enabled, triggering the start up of the airbag module. When the ignition key is inserted and turned as the car is to be started, a high inrush current may be drawn by the empty (i.e. discharged) reserve capacitor. Customers require an inrush current phase which is fast and controlled. However, serious problems have been reported in the past which are linked to inappropriate handling of the inrush current, even resulting in inadvertent deployment of the airbag when allowing the reserve capacitor to be charged in an uncontrolled manner.
With remotely controlled door locks entering the automotive market many car manufacturers choose to turn on the airbag module prior to the ignition key being inserted into the ignition, namely when the remote control button unlocking the car doors is pressed. This feature may require a so called “wake switch” which permits the airbag module to be initiated prior to turning on the car in the traditional way via its ignition switch. However, the “wake switch” in off-state may cause a very low quiescent current consumption when in sleep mode.
The two previously described requirements are typically provided separately from one another in airbag modules. The capacitor inrush current limiter may be provided in the form of an NFET (n-channel field effect transistor) with a current shunt resistor. The current flowing through the NFET is monitored via the shunt resistor and an overcurrent regulator may be used to control the gate of the NFET such that the NFET is driven into a more or less conducting state in correspondence to the current flowing therethrough. However, this solution requires an additional shunt resistor and an NFET gate driver including an overcurrent regulator. Both components are space consuming and costly. Instead of the separate additional shunt resistor, a special sense-NFET may be used which is provided with an internal current sensing functionality. Those approaches may have the further disadvantage that the NFET acting as a linear controller may get and therefore has to dissipate more power in the event of a high inrush current. Furthermore, as the current is limited, the main microcontroller of the airbag module may start its operation with a delay on the order of 100 milliseconds or more.
A further setup configured to provide capacitor inrush current limitation may include a current limiting resistor coupled between a reference potential and one side of the reserve capacitor, wherein a switch, e.g. a NFET, may be coupled in parallel to the current limiting resistor. The current limiting resistor is bypassed with the NFET during normal operation of the airbag module, i.e. after the inrush current phase when the car is already running. In a yet further setup, a boost converter is arranged between the battery and the reserve capacitor. A current limiting resistor is arranged between the boost capacitor of the boost converter and the reserve capacitor, wherein a diode is coupled in parallel to the current limiting resistor in order to provide a reverse path in case the battery is lost and the whole infrastructure has to be provided with energy from the reserve capacitor. In this scenario, the boost capacitor and the reserve capacitor have to be provided as separate entities, adding to the space and cost requirement of the arrangement.
The wake switch functionality may be implemented by providing a wake switch, for example a PFET (p-channel FET) between the battery and the airbag module, wherein the wake switch may be turned on and off by means of a respective signal from the car's electronics, for example by means of a respective CAN (controller area network) signal. The PFET may be replaced by an NFET, which is roughly only half the size of a PFET, but requires a gate voltage lying above the source voltage such that a charge pump may have to be used. However, both setups are not provided with a soft start function.
In a further approach, the wake-switch functionality and the capacitor inrush current limitation functionality may be provided in one common integrated concept. This concept is based on an intelligent boost converter rectifier diode, which consists of a NFET based face to face configuration to eliminate the existing body diodes and to achieve an “ideal switch” like a relay. The two rectifier diodes provide isolation of the reserve capacitors from the battery in sleep mode (off-state) and further an inrush current limitation for power-on of the main operating module and rectifying functionality for the boost converter operation.